So, we’re in the midst of a pretty bad snowstorm here. Outside my window are a few trees with berries on them. As far as I know, birds have never been that interested in any of the trees - they’re mainly occupied by tons of squirrels. A few hours ago, glancing out the window, I saw swarms of robins perched on two or three of the trees! All together there were 50 - 100 of them, maybe more. After a while they flew away, but looking out again now, I see they’ve come back. They seem to flutter around a bit, perch on the branches and eat some berries. The wind and snow is fairly harsh outside, so I have no idea why they’re doing it. Is it common for robins or other birds to come outside in snowstorms? Is there any explanation for what’s happening, or am I living in a Hitchcock film?
Some of us northerners would think that by default “robins in a snowstorm” means you’re in a Hitchcock film.
Though it’s hardly consolation, it would also make a good title for something by Tennessee Williams.
They’re probably stocking up in preparation for making it through the storm. They want to get as much into their bellies as possible (and lay on some fat if they can) in case it becomes entirely impossible to forage over the next few days.
Think of it like people raiding the supermarkets to stock up when there is a hurricane warning.
My WAG is that the berries are edible, but not a preferred food item, so they resort to eating those particular berries when other food supplies are exhausted or inaccessible. Think of some food item you dislike but that is not going to make you ill; you just don’t like it. Say … boiled Brussells sprouts, for a lot of people. If it’s a choice between that and, say, a hot fudge sundae, you’ll probably go for the ice cream. But if it’s a choice between the boiled sprouts and starving to death … most folks will eventually hold their nose and eat the damn veggies already.
The alternative is for them to starve to death. They’re tiny creatures with fast metabolisms, so they can’t go for weeks without food the way a healthy adult human can. Don’t forget that they can’t put on more than a certain limited amount of excess fat; too much and they affect their own ability to fly.
Fair enough. I think this is probably the answer. If it matters, after researching a bit, it turns out the berries are winter berries, which robins sometimes eat when the ground is too cold to push a beak into (or, you know, covered in snow).
That was pretty much my reaction. I’ve seriously almost never seen any birds near these trees, then they decide to swarm them en masse in a snowstorm, then disappear. At least my cat was entertained looking out the window. 
I’m no expert but I have heard that birds will feast when they sense a storm is coming.
Anecdote: Last Saturday I filled a large birdfeader with seed. The birds flocked to it and ate about 2/3 of the seed in it in two days. The lawn was full of robins. Today it snowed heavily and there were hardly any birds to be seen. I have to presume they were filling up before the storm.
Can I just say that “flock of robins” makes no sense to me here in the UK.
Our version of the robin (the real version of course) is territorial to the point of psychopathy. Really, they are vicious little sods and will try to kill anything that catches its manic little eye.
Watching them try to take on the whole bird-feeder gang in our back garden is endless, bloodthirsty fun. Cute little bugger though.
Ninjad:)
I have sat and watched my garden robin see birds twice its weight off. I sometimes wonder how they ever get together long enough to make baby robins.
Some berries are poisonous until at least some heavy frosts, if not until late winter. Holly is one of these. This may be part of why they were passed up before.
As opposed to curling up indoors with a good book and a cup of hot cocoa?
What do you mean?
In my experience, robins are outside pretty much all the time, and when they’re not, there’s someone trying to chase them out with a broom.
Cute little bird! 
The American version is pretty territorial during mating and nesting season, but in the winter months (non-breeding time) they can sometimes accumulate into big crowds, especially if there’s plenty of food.
I wonder if the OP thinks that robins hibernate and then for some reason wake up during a snowstorm.
US folklore sees the robin as a harbinger of spring, but many American robins don’t migrate at all. They overwinter in the harsh cold. The robin’s preferred food is larval insects and earthworms, but they’ll eat berries and seeds.